Well it mostly depends on how comfortable you are with the class mechanics and how good you are tracking ES. In a nutshell this is my thought process with MB in a fight.

-Get and orb and cast MB to get ES up asap.

-IF MB comes off CD and I have no orb AND there is at least 8 secs left on ES (you generally get 2 MB's for every duration of ES roughly) then cast MB. If no orbs AND there is less than 8 secs left on ES then wait for orb.

-IF I chose to wait for an orb (or I cast a MB with no orb and MB is now on CD) and an orb doesn't proc and ES is down to 3 secs, recast VT and DP, then cast MB.

-Start over at the top.

... I hope that makes sense. If it's confusing I can smush it into a flowchart

I use MB on cooldown if i have at least one orb up, if not i'll wait... The way described above is probably mathematically better, but this way is easier and you can pay more attention to other stuff going on around you.

I have done various tests on this to see if there was an advantage one way or the other. Honestly there doesn't seem to be a big difference either way. Waiting for 1 orb to cast MB is in my opinion the better option as that ensures there will be hardly any downtime on ES and it also makes the rest of the rotation a little easier to manage. I noticed that when I cast MB on CD I would lose the ES buff a lot more which is potentially very bad depending. Either way will work just fine though from my tests. This will possibly change with 4pc T-12 since MB will do more damage.

1 orb + MB off CD = CAST
0 orb + MB off CD + more than 6.5sec left on ES = CAST
0 orb + MB off CD + less than 6.5sec left on ES + more than 8sec left on BOTH VT and DP = CAST (debatable topic)
0 orb + MB off CD + less than 6.5sec left on ES + less than 8 sec on either VT or DP = DONT CAST*

*You can still choose to cast but by doing so you must prematurely refresh your VT and DP before ES falls off. Although this will allow your VT and DP to retain the ES effect, SW:P and MF will not.

Mind Blast is something that should be used frequently, usually used with at least 1 shadow orb to refresh empowered shadows. Whenever you refresh empowered shadows, you can afford to cast the next mind blast that's coming off CD without any orbs, since by that time there is still enough time to wait until the mind blast after that for refreshing purposes without jeopardizing the empowered shadow's uptime. However by doing so, you are slightly gimping your chances to accumulate any shadow orbs from mind flay, since the time spent casting the 0 orb mind blast could have been 2 ticks of mind flay, hence 2 chances to generate an orb. However in the long run these 2 extra chances are relatively quite small, and usually taking that risk (as opposed to playing safe) to cast the 0 orb mind is more worth it in terms of overall dps. However again there is an exception to this risk in the circumstance where dots are close to falling off soon (less than 8 seconds is a reasonable estimation). In that case I'd rather play it safe and NOT cast that 0 orb MB because I don't want to risk refreshing those dots in case I lose my empowered shadows at the same time.